Storage devices commonly implement data redundancy operations (also referred to as mirroring or back-up operations) for data recovery. Although entire storage devices may be copied onto secondary or redundant storage devices periodically as a means of backing up the data, more sophisticated data redundancy operations only record the changes to redundant storage devices.
Synchronous data redundancy operations record write operations at a primary storage device and at the redundant storage device before the operation is completed. Asynchronous data redundancy operations record write operations at a primary storage device during normal operation, and then update one or more redundant storage device with any changes at a later time. For example, the redundant storage device may be updated with changes during off-peak hours to reduce delays during normal operation.
These techniques require that any changes to the primary storage device be transferred to the redundant storage device in the same order in which the changes occurred at the primary storage device so that the redundant storage device is a true copy of the primary storage device at a definable point in time. In distributed storage arrays, multiple transactions may arrive simultaneously at different storage cells, making it difficult to know which transactions arrived first, given the skew between timestamps on different storage cells and variations in network performance. In addition, the first transaction to arrive at a storage cell may not be the first to complete.